<!DOCTYPE html>
<html>

	<head>
		<meta charset="UTF-8">
		<title></title>
		<style type="text/css">
			/*第一种写法*/
			/*.triangle_border_nw {
				width: 0;
				height: 0;
				border-width: 30px 30px 0 0;
				border-style: solid;
				border-color: #6c6 transparent transparent transparent;
				margin: 40px auto;
				position: relative;
			}*/
			/*第二种写法*/
			.triangle_border_nw {
				width: 0;
				height: 0;
				border-width: 0 0 30px 30px;
				border-style: solid;
				border-color: transparent transparent transparent #6c6;
				margin: 40px auto;
				position: relative;
			}
		</style>
	</head>

	<body>
		<div class="triangle_border_nw"></div>
	</body>

</html>